1. Field of the Invention
The present invention relates to a photoemission electron microscopy and, more particularly, to a light source system for a photoemission electron microscopy. Further, the present invention relates to a measuring method using a photoemission electron microscopy having the light source system. The photoemission electron microscopy (PEEM) is capable of observing the state of a specimen surface by utilizing a photoemissive effect. The principle of a light source system for an X-ray photoemission electron microscopy (XPEEM) categorized as a different device because of radiation of different kind of electromagnetic wave is the same as the light source system for the photoemission electron microscopy. In this sense, the light source system used in the present invention comprises that for an X-ray photoemission electron microscopy (XPEEM).
2. Description of the Related Art
A measurement using a photoemission electron microscopy (PEEM) is carried out by observing a contrast image of a specimen surface such as a metal formed from low energy electrons generated when the specimen surface is irradiated with light from a mercury lamp, light from a deuterium lamp, and the like. The principle of this measurement is utilization of a phenomenon in which when the specimen surface is irradiated with light having energy equal to or higher than the work function of constituent atoms of the specimen surface, photoelectron emission relating to the work function is caused by the photoemissive effect. If an atomic structural non-uniformity or adsorption molecules and the like exist in a specific area on the specimen surface, local variation in the amount of generated photoelectrons due to such a condition is obtained as a contrast image corresponding to the work function distribution.
Also, when a polarizing filter is inserted in a light source system constituted between a light source for excitation and a specimen surface, a measurement of a magnetic domain distribution using magnetic dichroism can be made.
As this kind of microscopic apparatus, inventions of multi-emission electron microscopic apparatuses for chemical analysis using a photoemission electron microscopy (PEEM) or an X-ray photoemission electron microscopy (XPEEM) have been disclosed (see Patent Document 1 for example).
[Patent Document 1] Japanese Patent Laid-Open No. 2000-215841 (page 2, FIG. 1).
In some cases, mercury lamp light (5.1 eV), deuterium lamp light (6.9 eV) or light having the same energy is used as an excitation light source for a photoemission electron microscopy (PEEM). FIG. 1 shows the outline of a light source system between from an excitation light source to a specimen surface in a conventional photoemission electron microscopy (PEEM).
Referring to FIG. 1, light rays in an ultraviolet wavelength region emitted from a light source 1 formed of a mercury lamp are made parallel through a condenser lens 2 formed of a pair of aspherical lenses, exit outside a light source housing 4 after passing through an additional lens 3, and are irradiated on a specimen surface 7 through a transmission window 6 corresponding to an incident path of a vacuum chamber 5 maintained at 10−7 Pa or lower by an ultra high vacuum pumping mechanism (not shown). At the time of irradiation with the ultraviolet rays, photoelectron emission related to the work function is caused by the photoemissive effect, as described above. Photoelectrons thereby emitted form an image on a fluorescent screen through an optical system which is constructed with a cathode lens, a contrast aperture, a projection lens, a multichannel plate (MCP) (each not shown) to enable imaging on the fluorescent screen with low energy electrons emitted.
FIG. 2 shows the result of analysis of the amount of collection of light with which a 200 μm square specific area 8 formed on the specimen surface 7 was irradiated using the light source system shown in FIG. 1. This analysis was made by computer simulation. As can be visually recognized from FIG. 2, the collecting effect of the light source system shown in FIG. 1 is low and the rate of collection on the specific area 8 is about 0.1% of the total amount of light emitted from the light source 1 shown in FIG. 1.
This is the reason that general-purpose industrial lamps for uniformly irradiating a comparatively large area are used as various ultraviolet light sources of actual models. Therefore, it is difficult to obtain irradiation light with a sufficiently high luminance on the specimen surface in the conventional system. In particular, in a case where a diffraction grating for separating and sweeping irradiation light in measurement of surface distribution of a work function, or the above-mentioned polarizing filter for measurement of a magnetic domain distribution is added to the optical path as light source system components, a further reduction in luminance is caused as a considerable hindrance to real-time measurement at the desired resolution. That is, in order to compensate the reduction in luminance, a longer time is required for measurement, which makes it difficult to carry out real-time measurement.
On the other hand, it arises another problem to increase the luminance of the light. In the conventional system, irradiation light reaches a wide area extending outside the specific area having a diameter of about 200 μm. Therefore, when the luminance is increased, gas emission from a portion of the specimen surface other than the measured portion and from the specimen holder heated by light occurs to cause a reduction in the degree of vacuum in the vicinity of the specimen surface. If a contamination occurs such that the clean environment in the vicinity of the specimen surface is impaired as described above, it is impossible to perform observation, for example, for the purpose of real-time measurement of gas adsorption to the specimen surface.